Epoxy resins are high performance materials used in a wide variety of applications including protective coatings, adhesives, electronic laminates (such as those used in the fabrication of computer circuit boards), flooring and paving applications, glass fiber-reinforced pipes, and automotive parts (including leaf springs, pumps, and electrical components). In their cured form, epoxy resins offer desirable properties including good adhesion to other materials, excellent resistance to corrosion and chemicals, high tensile strength, and good electrical resistance. Two challenges associated with the use of epoxy resins are the brittleness of the cured epoxy resins and the need to heat many curable epoxy compositions enough to prepare, blend and shape them, but not so much as to cure them prematurely.
Poly(arylene ether) resin is a type of plastic known for its excellent water resistance, dimensional stability, and inherent flame retardancy. The addition of a poly(arylene ether) to an epoxy resin can reduce the brittleness of the epoxy resin. For example, U.S. Pat. No. 4,912,172 to Hallgren et al. describes a composition including a specific epoxy resin and a polyphenylene ether having a number average molecular weight of at least about 12,000. However, relatively high temperatures are required to dissolve the polyphenylene ether in the epoxy resin, and premature curing of the epoxy resin is a risk at those high temperatures.
As another example, U.S. Pat. No. 5,834,565 to Tracy et al. describes compositions including an epoxy resin and a poly(arylene ether) having a number average molecular weight less than 3,000. The low molecular weight poly(arylene ether) is easier to dissolve in the epoxy resin than higher molecular weight poly(arylene ether)s. However, the products obtained on curing these compositions are not as tough as those prepared with higher molecular weight poly(arylene ether)s.
The poly(arylene ether)s in the Hallgren and Tracy patents are monofunctional, i.e. they possess one terminal phenolic group. U.S. Pat. No. 7,655,278 to Braidwood et al. describes compositions including an epoxy resin and a poly(arylene ether) having two terminal phenolic groups. These compositions are easier to dissolve in the epoxy resin, and can advantageously be dissolved at 10 to 40° C.
Despite the higher solubility of bifunctional poly(arylene ether)s over monofunctional poly(arylene ether)s and the known reactivity of phenols with epoxy resins, both monofunctional and bifunctional hydroxyl-terminated poly(arylene ether)s are relatively unreactive with epoxy resins at or near room temperature. The base catalyzed reaction of the terminal phenolic groups with epoxy resins is a slow reaction, which can require high temperatures and long times for complete curing. This lower reactivity is problematic in diamine cured epoxy compositions. In general, stoichiometric or close to stoichiometric quantities of diamines are used to cure epoxy systems. The reactivity of epoxy resins with diamine hardeners is significantly higher than with phenolic compounds. With diamine hardeners, curing can occur under ambient conditions. Therefore, in ternary resin compositions comprising poly(arylene ether), epoxy resin, and diamine hardener, the epoxy resin reacts preferentially with the diamine hardener. There is little or no reaction of the epoxy resin with the poly(arylene ether), and there is little or no incorporation of the poly(arylene ether) into the thermoset matrix formed by reaction of the epoxy resin and the diamine hardener. The presence of unreacted poly(arylene ether) can have adverse effects on the cured epoxy resin composition, such as dual phase morphology, poor solvent resistance, and reduced impact strength.
One solution to the low reactivity of the phenolic end groups of the poly(arylene ether) is to “up-stage” the poly(arylene ether). Up-staging is the partial reaction of poly(arylene ether) with epoxy resin at elevated temperatures with or without a catalyst. Temperatures for up-staging can be as high as 250° C., and the reaction can take several hours. Not every epoxy resin formulator has the equipment or time to do up-staging.
Another solution to the low reactivity of the phenolic end groups of the poly(arylene ether) is to use poly(arylene ether) terminated with glycidyl ether groups. Poly(arylene ether) terminated with glycidyl ether groups are disclosed, for example, in U.S. Pat. No. 7,276,563 B2 to Ishii et al., U.S. Patent Application Publication No. US2004/0214004 A1 of Amagai et al., and C.-T. Su, K.-Y. Lin, T.-J. Lee, and M. Liang, European Polymer Journal, volume 46, pages 1488-1497, 2010. However these polymers are produced using a large (e.g., about 30-fold) stoichiometric excess of epichlorohydrin. The large excesses of epichlorohydrin are used to minimize copolymerization of the epichlorohydrin with the poly(arylene ether) intermediate. Epichlorohydrin is not an environmentally friendly chemical. Thus its use in a manufacturing process, especially when it has to be used in such large excesses beyond the amount that reacts, is undesirable from industrial hygiene and environmental viewpoints.
In addition to the above problems with the process for making prior art glycidyl ether terminated poly(arylene ether)s, poly(arylene ether)/epoxy compositions can have a two-phase polymer morphology, poor solvent resistance due to solvent extraction of unreacted poly(arylene ether) that is not incorporated into the epoxy resin matrix, and poor impact strength.
In view of the above problems associated with prior art poly(arylene ether) epoxy compositions, there remains a need for an epoxy-terminated poly(arylene ether) that is highly reactive in epoxy-containing curable compositions, especially epoxy/hardener resin compositions, while at the same time maintaining good solubility in the epoxy resin composition. There also remains a need for an epoxy-terminated poly(arylene ether) that does not require upstaging prior to use, and does not require the use of epichlorohydrin in its manufacture. In terms of epoxy resin compositions, there remains a need for poly(arylene ether)/epoxy/compositions having a single phase polymer morphology, improved solvent resistance, and improved impact strength.